if you need a visa, then you must get it before you arrive. Otherwise you'll get a visa exempt stamp for 60 days when you arrive (assuming you have American passport?). Once you're here, you can sort out a longer-term visa option or search for a visa agency.
Application fee is 10,000 baht. And it's very easy to do yourself, there's no need for a visa agent for DTV. This sounds like a total rip off and possible scam.